(1) Field of the Invention
This invention relates to a method of removing foil shadows of a synchronous grid which removes scattered radiation of a radiographic apparatus, and a radiographic apparatus using the same.
(2) Description of the Related Art
Conventionally, an X-ray apparatus includes an X-ray tube, and an X-ray detector opposed to the X-ray tube. The X-ray detector has a grid disposed adjacent an X-ray incident plane thereof. The grid reduces image quality degradation due to scattered X-rays, but on the other hand, grid foil shadows are superimposed on radiographic images.
An FPD (Flat Panel Detector) has come to be used widely as the X-ray detector in recent years. The FPD can improve the spatial resolution and X-ray sensitivity of radiographic images, and for this and other reasons its use is spreading at a rapid rate. However, as improvement is made in the spatial resolution and X-ray sensitivity of the X-ray detector, the grid foil shadows appear clearly on radiographic images, which are obstructive to interpretation of the radiographic images. In order to remove these grid foil shadows from the radiographic images, it is known to remove the shadows by image processing using frequency conversion. See Patent Document 1 (Japanese Unexamined Patent Publication No. 2000-83951 (paragraphs “0033”-“0036”)), for example.
Patent Document 1 describes what is called a fixed grid which is fixedly attached to the X-ray detector although changeable to one different in grid intervals. Besides the above fixed grid, there is a moving grid. The moving grid is moved in a direction perpendicular to grid stripes synchronously with X-ray irradiation to prevent a fixed pattern of the grid from appearing on radiographic images. Although the fixed pattern of the grid does not appear on radiographic images, the moving grid has problems of requiring a complicated moving mechanism and lowering detection efficiency. Such grids are constructed of an alternate arrangement of metal foil strips consisting of an X-ray absorbing material such as lead, and interspacers consisting of aluminum or carbon fiber which does not easily absorb X-rays. However, these interspacers absorb a certain quantity of X-rays, which leads to a sensitivity lowering of desired X-ray images. So, a synchronous grid has been proposed as a solution to these problems. See Patent Document 2 (Japanese Unexamined Patent Publication No. 2002-257939 (paragraphs “0018” and “0019”, and FIG. 1)).
This synchronous grid has grid foil strips arranged so that grid foil shadows fall in middles of detecting pixels of the FPD. More particularly, the grid foil strips are arranged as inclined such that each has flat surfaces thereof aligned to a straight line extending between a focus of the X-ray tube and an X-ray detecting plane of the FPD.
Patent Document 3 (Japanese Unexamined Patent Publication No. 2008-232731 (paragraphs “0001”-“0003” and “0007”)), for example, describes a specific method of manufacturing a synchronous grid. This method excludes the interspacers members to provide layers of air, thereby to obtain X-ray images of improved sensitivity.
However, the conventional example with such construction has the following problems.
For reasons of manufacture of the grid foil strips and construction for aligning the grid foil strips, the synchronous grid has a certain distortion of the linear grid foil strips and a minute shift in their positions of arrangement. Further, since the grid foil strips of the synchronous grid are higher than those of the other type grids, the foil shadows of the synchronous grid are susceptible to influences of the distortion and shifting of the grid foil strips. Distortion and shifting occur also with the grid foil shadows, which are caused by the distortion and shifting of the grid foil strips. As a result, variations in measurements of the foil shadows will occur to different lines of the grid foil shadows, and density variations will occur to the grid foil shadows. There is a drawback that, even if frequency conversion is used for removing the grid foil shadows, the grid foil shadows in a longitudinal pattern cannot fully be removed. The grid foil shadows failing to be removed will become artifacts on radiographic images after the grid foil shadows are removed therefrom.
In an X-ray apparatus having a C-arm, the heavy X-ray tube and FPD are mounted at opposite ends of the C-arm. Thus, subtle bending of the C-arm will occur with movement such as rotation of the C-arm, thereby causing a shift between the FPD and the focus of the X-ray tube. This shift is in the order of 2 mm, for example, but the grid foil shadows on the FPD will also move, and hence a problem that the grid foil shadows cannot fully be removed.
In order to solve the above problems, Applicant has proposed the following technique (International Application PCT/JP2010/003221).
According to this technique, pixels free from influences of grid foil shadows are first extracted from a fluoroscopic image, and an interpolation process is carried out based on detection signal values of these pixels, to obtain an approximate fluoroscopic image without influences of the grid foil shadows. Next, a grid foil shadow image which is an image of only the grid foil shadows is obtained based on a difference between the fluoroscopic image and the approximate fluoroscopic image. Further, the grid foil shadow image is averaged to obtain a foil shadow standard image inhibiting variations in the grid foil shadows due to random errors such as quantum noise. Then, based on the foil shadow standard image and the fluoroscopic image, the grid foil shadows are removed from the fluoroscopic image. This is a technique intended to obtain, through such processes, a fluoroscopic image with no grid foil shadows appearing thereon.
It is important for the above proposed technique how the pixels free from influences of the grid foil shadows should be extracted. However, under the influence of random quantum noise occurring with X-rays, pixels influenced by the grid foil shadows can be extracted in error. Then, since the approximate fluoroscopic image has low accuracy, accuracy of the grid foil shadow image also becomes low. There arises a problem that it is impossible to remove the grid foil shadows from the fluoroscopic image ultimately with high accuracy, with artifacts remaining to impart influence.